1. Field of the Invention
The invention relates to an air/fuel ratio closed loop fuel control of an internal combustion engine equipped with an Exhaust Gas Oxygen (EGO) sensor.
2. Prior Art
Numerous closed loop fuel control systems are known for controlling the air/fuel ratio to a predetermined (usually stoichiometric) value in response to an EGO sensor. The EGO sensor normally generates an output which changes sharply in amplitude at stoichiometry as a detected oxygen concentration crosses stoichiometry. More particularly, the output of the sensor is high when air/fuel ratio is rich, and the output is low when air/fuel ratio is lean. The output is compared with a reference level that corresponds to a desired air/fuel ratio in the vicinity of stoichiometry to generate an error signal indicative of the deviation from the desired air/fuel ratio. The error signal is assigned the value of 1 for a rich air/fuel ratio, and the value of -1 for a lean air/fuel ratio. The error signal is fed into a fuel controller which is usually a Proportional and Integral (PI) controller. In response to the error signal, the PI controller originates a familiar limit cycle control.
The output of the EGO sensor can be contaminated with a high frequency noise which appears as short duration spikes superimposed on the EGO signal. The noise frequency can correspond to an engine firing frequency, and the sources of noise can be injector fuel flow maldistribution, incomplete combustion, engine ignition system (spark firing), and the like. These spikes (of either induced electrical noise or air/fuel ratio deviation) may cause a false triggering of the error signal and disrupt a limit cycle. Under certain unfavorable conditions, such as a low load during deceleration, fuel control system may lock into this high frequency noise and start switching with the high frequency. It leads to a temporary loss of a fuel control which will recover only after substantial air/fuel ratio errors.
To alleviate this problem, error detection incorporates different noise suppression techniques. One widely used technique includes hardware or software low pass filter (see U.S. Pat. No. 4,149,502 issued to Johnson et al). Another technique described in U.S. Pat. No. 4,811,557 issued to Okumura et al uses time delay to acknowledge the crossing of stoichiometry to make sure that it is not spurious noise induced change. Yet another technique may incorporate a comparator with hysteresis. A common disadvantage of all these approaches is an additional time delay which decreases frequency of the limit cycle.
The object of the present invention is to improve fuel control system noise protection without introducing permanent time delays thus increasing the frequency of the limit cycle.